1. Field of the Invention
The present invention relates to a coordinates input apparatus and, more particularly, to a coordinates input apparatus for detecting coordinates of an instructed point from a vibration propagation time on a vibration propagation plate and having a structure in which a vibration-proof material is attached to an edge portion of the vibration propagation plate.
2. Related Background Art
Hitherto, as an apparatus for inputting, a hand-written character, figure, or the like to a processor such as a computer, a coordinates input apparatus using various kinds of input pens, a tablet, and the like is known. According to this kind of apparatus, image information consisting of the input character, figure, and the like is output to a display device such as a CRT display and a recording device such as a printer.
As a method of detecting the coordinates on a tablet in such a kind of apparatus, the following various kinds of methods are known.
1) A method whereby a change in resistance value of a sheet material which is arranged so as to face a resistance film is detected.
2) A method whereby electromagnetic or electrostatic induction of a conductive sheet or the like which is arranged so as to face a coil (an inductor) is detected.
3) A method whereby ultrasonic vibration which is propagated from an input pen to a tablet is detected.
According to the above methods of 1) and 2), since a resistive film and a conductive film are used, it is difficult to form a transparent tablet. On the other hand, according to the method of 3), since the tablet can be made of a transparent material such as acrylic plate or glass plate, it is possible to construct an information input and output apparatus in which the input tablet is overlaid on a liquid crystal display device or the like and this apparatus can be used to give the operator the feeling as if he is writing an image onto paper.
On the other hand, according to the foregoing ultrasound method, various kinds of peculiar problems exist because the coordinates are detected through vibration.
First, according to a method whereby a surface wave is generated on the tablet and this wave is used to detect the coordinates, a reflected wave is generated by a scratch on the vibration propagation plate of the tablet or an obstacle put thereon, so that the coordinates detecting accuracy deteriorates. On the other hand, to avoid the problem of a scratch on the vibration propagation plate, a method whereby the vibration is propagated in air instead of a solid is also considered. However, this method also has a problem in accuracy since the vibration propagation characteristic is changed by an obstacle existing on the propagation path.
Further, a method whereby a plate wave, i.e. an elastic wave propagated in the plate, is used is effective with regard to interference from a scratch or obstacle on the propagation plate. However, since a velocity distribution occurs, there is a problem in that an error is caused in dependence on a vibration wavelength in the detection based on a certain threshold value.
On the other hand, in a method whereby the elastic wave vibration is propagated on the vibration propagation plate, a reflected wave is caused at the edge of the vibration propagation plate. Thus, there is a problem in that if the reflected wave is synthesized with the direct wave, a distortion occurs in the detected wave form and the detecting accuracy deteriorates.
To prevent this problem, there is considered a structure such that the peripheral portion of the vibration propagation plate is supported by a vibration-proof material made of polymeric materials or the like. Such a conventional method has a problem such that when the vibration is input at a position near the vibration-proof material, the detection level remarkably decreases, so that the coordinates detecting accuracy also deteriorates.
On the other hand, such a coordinates input apparatus has a structure such that the edge portion of the vibration propagation plate is supported by the vibration-proof material in order to prevent errors from being caused in the detection by a vibration sensor due to the reflected wave of the input vibration at the edge of the vibration propagation plate. As used in the present specification, the term "vibration-proof material" means a generally non-resonating, dampening material effective to suppress or substantially attenuate vibration thereat.
The conventional vibration-proof materials are mainly developed to prevent such noises and are mainly classified as materials used as a countermeasure for sound in the air and materials used as a countermeasure for sound in a solid. Therefore, the latter material is used in the case where the conventional vibration-proof material is used in the foregoing application. As the conventional vibration-proof materials used as the countermeasures for sound in the solid, a vibration-proof sheet, a paint, or the like for a thin plate as shown in FIG. 1A is known.
In FIG. 1A, reference numeral 8' denotes a vibration plate whose vibration is to be suppressed, and 7 indicates a vibration-proof sheet. The vibration plate 8' is made of a thin steel plate, a metal plate such as an aluminum plate, a resin sheet, a glass sheet, or the like.
In this structure, by adhering the vibration-proof sheet 7 onto the vibration plate 8' which vibrates, the vibration of the plate 8' can be reduced by using the vibration attenuation of the sheet 7 and the noise can be reduced.
As examples of materials for the vibration-proof sheet 7 which are commercially available, there can be mentioned polyvinyl chloride resin, atactic polypropylene, polyethylene-vinyl acetate, styrene butadiene rubber, silicone rubber, cement paste, or the like. Further, it is also possible to use the material which is obtained by adding or mixing thereto plasticizer, stabilizer, softener, metal powder such as lead or iron, quartz sand, asphalt, or the like. The sheet 7 is formed in a sponge-like porous shape.
However, it is to be noted that the conventional vibration-proof sheet as mentioned above is mainly adhered to the whole vibrating plate, thereby suppressing the vibration of the whole plate.
Therefore, even if the vibration-proof material is attached to the periphery of a vibration propagation plate 8 as shown in FIG. 1B to prevent reflected waves at the edges of the vibration propagation plate as mentioned above, the reflected waves cannot be sufficiently reduced.
FIG. 1B is a diagram showing the vibration propagation in the case where the vibration-proof sheet is attached to the peripheral portion of the vibration propagation plate. FIG. 1C is a cross sectional view showing a state of reflected wave in a part of FIG. 1B.
In the diagram, reference numeral 8 denotes the vibration propagation plate; 7 is the vibration-proof sheet; 3 is a vibration pen serving as a source for applying the vibration; A is a wave showing the vibration which is propagated from a vibration adding point as a contact point between the vibration pen 3 and the vibration propagation plate 8; B is a reflected wave at the edge surface of the plate 8; and C is a reflected wave at the boundary surface where the vibration-proof sheet 7 is adhered. The waves A, B, and C are diagrammatically illustrated in FIG. 1C, respectively.
The conventional vibration-proof sheet 7 has a large vibration-proof effect to the vibration (also including the natural vibration, i.e., the resonant vibration) which is generated in the whole plate as mentioned above. However, as shown in FIG. 1B, a sufficient vibration-proof effect is not obtained for the vibration in the form of progressive waves which are propagated from the portion to which the vibration was applied in the area where the vibration-proof sheet 7 is not attached. Therefore, as shown in FIG. 1C, although the vibration is slightly attenuated in the portion D where the sheet 7 is attached, the reflected wave B at the edge of the vibration propagation plate cannot be sufficiently suppressed.
Further, by attaching the conventional vibration-proof sheet 7, the reflected wave C is newly generated from the boundary surface where the vibration-proof sheet is attached. Therefore, when the conventional vibration-proof sheet is merely attached to the peripheral portion of the vibration propagation plate of the coordinates input apparatus using the elastic waves, there are drawbacks in that the foregoing two kinds of reflected waves are generated and become noises when the direct wave from the vibration applying source is detected, so that the detecting accuracy deteriorates.